To achieve high recording density of a HDD (Hard disk drive), it is essential to achieve high field strength to enable recording into a medium having high thermal stability, and field distribution that does not induce degradation of a recording pattern of an adjacent track in a high track density recording. However, the above two points are generally contrary to each other. That is, in a head structure for generating a high magnetic field, field expansion is also large in a track width direction. On the other hand, with increases in track density, a problem of signal degradation in an adjacent track, which may occur during recording, begins to emerge. This is a phenomenon that even if a leakage field to an adjacent track (hereinafter, called fringe field) is equal to or lower than a magnetization reversal field of a recording medium, signal quality of the adjacent track is degraded as a result of repeated recording.
As an approach for preventing field leakage to an adjacent track while keeping a maximum field, a magnetic shield may be provided on a side face in a track width direction of a main pole. Japanese Patent Publication No. 2004-127480 (“Patent document 1”) and Japanese Patent Publication No. 2007-35082 (“Patent Document 2”) disclose a method where a nonmagnetic film and a soft-magnetic film are formed on each side face of a main pole having an inverted trapezoidal shape formed by ion milling. Japanese Patent Publication No. 2007-24120 (“Patent document 3”) discloses a method where as means of forming a main pole having an inverted trapezoidal shape in a manner of self-aligning with a side shield, a groove having a wedge-shaped section profile is formed in a magnetic film, and a nonmagnetic film and a magnetic film are formed in the groove.
In order to increase field strength, a method is considered, in which a sub pole is provided at a position retracted from an air-bearing surface in a direction perpendicular to the air-bearing surface at an end in a down track direction of a main pole. For example, Japanese Patent Publication No. 60-136015 (“Patent document 4”) and Japanese Patent Publication No. 2006-244671 (“Patent document 5”) describe a method where a magnetic film is added to a leading side of a main pole, respectively. However, generally in these methods, field expansion is caused with increase in field strength, resulting in increase in fringe field. Therefore, track density cannot be increased, and consequently high density recording cannot be achieved.